Patent Literature 1 discloses a communication system for cryptographic communication between a mobile device carried by a user of a vehicle and an onboard apparatus mounted on the vehicle.
The communication system according to Patent Literature 1 includes the mobile device embodied as a smart key used for locking and unlocking a door, and the onboard apparatus embodied as a door lock device configured to lock and unlock the door. The smart key and the door lock device in the communication system according to Patent Literature 1 store multiple cryptographic communication keys issued from a key issuing apparatus configured to communicate with the smart key and the door lock device via a communication network. Furthermore, the smart key and the door lock device execute cryptographic communication with use of one of the stored keys, and specify a key matched in both of the smart key and the door lock device by switching the used key until succeeding in cryptographic communication. According to Patent Literature 1, the key used for cryptographic communication is updated in accordance with such a method while the smart key and the door lock device do not constantly communicate with the key issuing apparatus, thereby achieving security of the cryptographic communication.